Being that tastes are subjective. This is a difficult quesiton to answer. In most of the forums, it is suggested that

Tobacco - Start at 1% for most flavors ( I prefer to dive right in at 10%)
Candy - Start at 5%
Drink - Start at 3%
Fruits - Start at 5%

But it also depends on the flavoring manufacture. I mostly start by smelling the flavoring. If it is very strong, Then I start at 5%. If weak (Vanilla) then it jumps up and I start at 10%. Check out the Charts for more infromation.